1. Field of the Invention
The present invention relates to a hydraulic drive system of an injection molding machine, equipped with a fixed-displacement hydraulic pump and a servomotor for driving the hydraulic pump.
2. Description of the Related Art
Japanese Patent Application Laid-Open (kokai) No. 63(1988)-23002, for example, discloses a conventional hydraulic drive system which includes a fixed-displacement hydraulic pump and a servomotor for driving the hydraulic pump and in which the discharge rate and discharge pressure of the hydraulic pump are controlled through control of the rotational speed of the servomotor.
In such a hydraulic drive system, the rotational speed of the servomotor is controlled according to load volume. Thus, when such a hydraulic drive system is used in an injection molding machine and pressure control is performed in, for example, a pressure-holding step, the discharge rate of the hydraulic pump decreases due to decrease in load volume, and the rotational speeds of the servomotor and hydraulic pump decrease. The relationship between rotational resistance R and rotational speed V of the hydraulic pump is usually as shown in FIG. 2. Accordingly, in a region where the rotational speed V becomes extremely low; e.g., during the above-mentioned pressure control, the rotational resistance R becomes unstable, as represented by symbol Rs in FIG. 2. As a result, in such an unstable region, the controlled pressure in the pressure-holding step varies, causing impairment in quality of molded articles or in homogeneity of quality among molded articles.
In order to solve the above-mentioned problem, the present applicant has already proposed a hydraulic drive system disclosed in Japanese Patent Application Laid-Open (kokai) No. 11(1999)-105094. The proposed hydraulic drive system employs a relief circuit for releasing working fluid discharged from a hydraulic pump to an oil tank at the time of pressure control, so that the hydraulic pump can always be operated at a rotational speed higher than a certain rotational speed for exiting an unstable region in which the rotational resistance of the hydraulic pump changes unstably. In a control region where the rotational speed V of the hydraulic pump becomes extremely low; i.e., at the time of pressure control which is performed in, for example, a pressure-holding step and in which load volume decreases, working fluid discharged from the hydraulic pump is released to the oil tank through the relief circuit. Accordingly, the rotational speed V of the hydraulic pump is always sufficiently high to render the rotational resistance R stable, thereby avoiding variations in controlled pressure; i.e., enhancing stability in pressure control.
However, the hydraulic drive system disclosed in Japanese Patent Application Laid-Open No. 11(1999)-105094 involves the following problems to be solved.
Notably, some components in a hydraulic circuit which receives working fluid from the hydraulic pump generate an unstable region in operation of the hydraulic drive system. Particularly, a control valve having a pilot port brings the hydraulic drive system into an unstable or uncontrollable state during low-pressure or low-speed control. Specifically, the control valve requires a pilot pressure equal to or greater than a certain level (usually at least 0.4 MPa to 0.5 MPa). When the discharge pressure of the hydraulic pump decreases, there arises a failure to establish a sufficiently high pilot pressure. Thus, during the low-pressure or low-speed control, the control valve returns to its neutral position, thereby bringing the hydraulic drive system into an unstable or uncontrollable state.
The hydraulic drive system disclosed in Japanese Patent Application Laid-Open No. 11(1999)-105094 does not take direct measures to prevent the hydraulic drive system from entering the unstable control region, but takes indirect measures to avoid the unstable control region; i.e., release of working fluid, which is disadvantageous in terms of energy efficiency and control performance and causes complication of the hydraulic circuit with a resultant increase in cost and size.
An object of the present invention is to solve the problems which bring the control system into an unstable or uncontrollable state; e.g., the problem of a control valve returning to its neutral position, which problems would otherwise occur during low-pressure control in which an unstable region may be generated and to provide a hydraulic drive system of an injection molding machine capable of realizing low-pressure control and low-speed control, which have conventionally been considered to be difficult to perform.
Another object of the present invention is to provide a hydraulic drive system of an injection molding machine capable of reducing cost and size through simplification of a hydraulic circuit and of enhancing energy efficiency and control performance through elimination of release of working fluid.
To achieve the above objects, the present invention provides a hydraulic drive system of an injection molding machine which has a hydraulic power source equipped with a fixed-displacement hydraulic pump and a servomotor for driving the hydraulic pump and functioning to control the discharge rate and discharge pressure of the hydraulic pump through control of the rotational speed of the servomotor. In the hydraulic drive system, a pilot port of a control valve connected to a hydraulic actuator of the injection molding machine is connected to a supply line of working fluid discharged from the hydraulic power source, via a series circuit including a check valve for preventing backflow from the pilot port and a changeover valve.
Thus, even when the discharge pressure of the hydraulic power source drops below a required pilot pressure, the check valve functions to maintain a pilot pressure applied to the pilot port unchanged from a level which has been established before the discharge pressure drops, thereby preventing changeover of the control valve to a neutral position, or generation of an unstable control region, at the time of low-pressure or low-speed control.